Enzymatic biotransformations have been pursued extensively for many important chemical processing applications such as chemical production, drug synthesis, pollutant degradation, and petroleum refining because of their unparalleled selectively and mild reaction conditions. In many cases, however, low catalytic efficiency and stability of enzymes have been seen as barriers for the development of large-scale operations to compete with traditional chemical processes.
In practice, almost all large-scale industrial operations preferably employ immobilized enzymes because they afford easy recycling, feasible continuous operations, and simplified product purification. Many methods have been developed to incorporate enzymes into a variety of organic and inorganic solid supports, entrap enzymes in hollow fibers or microcapsules, and cross-link enzymes via covalent bonds. Among other factors, the structure of the support materials has a great impact on the performance of the immobilized enzymes. Nonporous support materials, to which enzymes are attached at the surfaces, are subject to minimum diffusional limitations. However, enzyme loading per unit mass of support is usually low. Alternatively, high enzyme loading can be achieved with porous materials such as membranes, gel matrices, and porous materials. Porous materials, however, suffer much greater diffusional limitation. For example, the value of effectiveness factor (η, which measures the ratio of apparent heterogeneous reaction rate to homogeneous reaction rate) for α-chymotrypsin entrapped in polyacrylamide hydrogel was reported to be ˜0.3; for α-chymotrypsin incorporated into hydrophobic plastics, η was below 0.1; for cross-linked α-chymotrypsin, η was below 10−3. Higher η values are possible for the same immobilized enzyme but used for nonaqueous reactions, mostly due to the relatively slower reaction rates involved there.
The reduction in size of support materials can effectively improve the efficiency of immobilized enzymes. In some cases, such at in the case of surface attachment on non-porous materials, smaller particles have been shown to provide relatively higher enzyme loading per unit mass. For porous materials, smaller particles are subject to much reduced diffusional resistance because of a shortened path of diffusion.
Many studies on the use of micrometer sized materials have been conducted. However, it has only been recently that even smaller scale materials have been studied. In these newer studies, nanoparticles have been used as carriers or supports for enzyme immobilization. The effective enzyme loading on nanoparticles can be very high, and a large surface area per unit mass is also available to facilitate reaction kinetics. It will be appreciated that the enzymes are attached to the nanoparticles.
While the use of nanoparticles provide excellent results in terms of balancing the contradictory issues of surface area, diffusion resistance, and effective enzyme loading, their ability to be dispersed in reaction solutions and their subsequent recovery for reuse are dauntingly difficult.
There is, therefore, still a need for immobilizing enzymatic catalysts by employing substrates that have the benefits of nanosized materials, e.g., have relatively high enzyme loading capability, have large surface area, and have minimal diffusional limitations, but yet are easily recoverable for reuse or continuous use.